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The footage was filmed from a dashcam mounted in a car and begins with that dashcam car in Lane 2.

A white Ford Fiesta filmed on the westbound M4 in Bristol

The westbound M4 at this point is already marked into two sets of two lanes - the two left hand lanes are for traffic exiting the M4 onto the M5 junction, the right two lanes for vehicles remaining on the M4.

The dashcam car begins to change lanes to the left, from Lane 2 to Lane 1. But almost immediately, a white car passes down the narrow left-hand side, on the hatched out-of-bounds side of Lane 1, undertaking the dashcam car at speed before rejoining Lane 1 just in time.

While the hard shoulder is in use at this point, there is a hatched area which is narrower than a regular lane, not to be used for breakdowns. As the M4 passes under the Trench Lane road bridge in Bradley Stoke, the hatched area disappears and there is nothing but the four lanes.

The white car manages to pull back onto the legal carriageway just as that hatched area narrows.

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But others pointed out that the white car is speeding - it is going considerably faster than the other cars travelling along the motorway at the time - and was travelling too fast to safely slow behind the dashcam car as it changed lanes in front of it - necessitating the dangerous move.